BGP sub autonomous system

When implementing confederations in a BGP implementation, you create what are known as sub-autonomous systems or sub-ASes. When confederations are used, an AS is subdivided into sub-ASes to make configurations and administration more manageable, especially for extensively large ASes.

When sub-ASes are in use, BGP updates will include them in the AS path. However, there are rules that govern when they appear and when they should not appear. This is specifically described in RFC 3065, in the section that deals with AS_PATH modification rules. Specifically, two attributes are used for this purpose:

  • AS_CONFED_SEQUENCE
  • AS_CONFED_SET

These attributes include the sub-AS numbers in the AS_PATH. However, when communicating with a BGP speaker located in a neighboring AS that is not a member of the current autonomous system confederation, the advertising speaker updates the AS_PATH attribute by removing the AS_CONFED_SEQUENCE and/or AS_CONFED_SET values completely.

In other words, when a BGP update is sent to another AS, the sub-ASes are stripped from the AS_PATH. This makes sense since those sub-ASes only have local significance within the AS that they subdivide. Their values are meaningless to other ASes.

https://datatracker.ietf.org/doc/html/rfc3065#section-6.1 https://networklessons.com/bgp/bgp-confederation-explained

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